Electronic wafers are usually manufactured using a layer-by-layer methodology, with each layer's manufacture including various etching and deposition processes, which are applied with the aid of a reticle and/or a mask. To create a complete layout on a wafer, the reticle is stepped sequentially over the wafer and at each step, the pattern of the reticle is transferred to the wafer.
The reticle itself is the end product of a complicated design process in which the entire layout of the wafer is determined and tolerances are set for the manufacture process. Nevertheless, it is often the case that the final manufactured wafer deviates from the design of the reticle. The reasons for the wafer being different from the design of the reticle may include defects in the reticle manufacture and unexpected interactions between the reticle design and the process used for the wafer manufacture. Therefore, reticles are typically inspected both by the mask shop manufacturing the reticle and at the semiconductor fabrication plant (Fab). According to current practice, reticles having non-repairable defects thereon are not shipped by the mask shop or are returned by the Fab, and never used. Due to the rapid reduction in design rules (the smallest resolvable feature size), manufacturing reticles is expected to become more difficult and the price of reticles is expected to rise. Therefore, it will become more desirable to use a reticle even if it has defects in its manufacture, providing however, that such defects do not invalidate the manufactured wafer.
One method of determining which defects cause problems in wafer manufacture is to simulate the physical processes which are involved in wafer manufacture. When a reticle is inspected after its manufacture, any detected defects are analyzed based on these simulations to determine if they will have an adverse effect on the wafer manufacture. If the adverse effect is within certain tolerances, the reticle is accepted. These simulations may also be used to generate a set of rules that govern which defects in a reticle are acceptable and which are not.
A critical problem with this approach is that the physical process must be understood in order to be simulated. Specifically, a simulation model of the stepper and fabrication process are first constructed. These models require expertise eof each stepper-fabrication combination, including, for example, the type of stepper used, the technology used to print the wafer and the development process A new model may be required for each new combination of stepper, exposure and other processing steps and/or parameters. In some cases, a physical understanding of the parameters of the process are not available. As a result, Fab technicians are often uncomfortable relying on a model instead of on the actual equipment used in the Fab.